Heretofore, there is a catalyst carrier for the purification of an exhaust gas from an automobile, e.g. a catalyst carrier for purifying an exhaust gas from a diesel engine as shown in FIGS. 1(a) and (b). As a typical example of such a catalyst carrier, there is a honeycomb type filter 100 by making each cell 101 as a path for an exhaust gas in a honeycomb shape from a porous silicon carbide sintered body having excellent heat resistance and thermal conductivity and alternately clogging these cells 101. This type of the honeycomb type filter 100 is connected to an exhaust side of a diesel engine and has a structure that PM (particulate matter) deposited on the filter, HC, CO and the like are removed by oxidation decomposition.
As such a carrier for the catalyst, it is known that a carrying layer made of γ-alumina is formed on a surface of a filtering wall (cell wall) 102 of a honeycomb-shaped heat-resistant carrier made of, for example, cordierite and a noble metal catalyst such as Pt, Pd, Rh or the like is carried on the carrying layer. For example, JP-A-5-68892 discloses a catalyst carrier obtained by adding and mixing γ-alumina with an inorganic binder and pulverizing them to obtain a fine powder slurry and then uniformly spraying the slurry onto a surface (wall face) of a honeycomb filter made of cordierite to form so-called wash coat alumina layer 103.
The alumina layer 103 formed by the conventional technique or wash coated (wash coat alumina layer) is shaped by a thin film uniformly covering the wall face of the filtering wall 102 as shown in FIG. 2(a) and has a fine pore structure as shown by a partial enlarged view in FIG. 2(b). The pore size in such a fine pore structure is mainly 20–500 angstrom and a specific surface area is usually 50–300 m2/g. And also, the alumina layer 103 acts as a catalyst carrying layer dispersingly supporting a catalyst such as a noble metal or the like on its surface, so that it is required to enlarge the surface area and have a certain thickness (about 50–100 μm).
However, the wash coated alumina layer 103 is small in the pore size and porosity and large in the permeation resistance, so that there is a problem that pressure loss considerably increases as compared with the carrier having no alumina layer.
Furthermore, the wash coated alumina layer 103 is poor in the adhesion property because it is uniformly coated onto the surface of the carrier as a filtering wall 102. Therefore, when the deposited ash is cleaned after the purification of the exhaust gas, the alumina layer 103 is feared to be simply peeled. And also, there is a problem that the wash coated alumina layer 103 is poor in the heat resistance because such a layer has a fine pore structure as mentioned above but is as small as 20–500 angstroms in the pore size and proceeds the sintering when being exposed to a higher temperature for a long time to cause a phase transformation into α-phase to lower the surface area. Furthermore, since the surface area is small, a distance between noble metal particles carried on the alumina is small and hence the lowering of the surface area is caused as the sintering proceeds and there is a problem of lowering the catalytic action itself.
As seen from the above, it is important that the surface of the carrier, particularly the surface of the alumina layer covering the surface thereof is maintained at the high temperature for the long time as an important property of the catalyst carrier.
It is, therefore, an object of the invention to propose a catalyst carrier being large in the pore size and porosity and small in the pressure loss though an alumina thin film is formed on the surface and a method of producing the same.
It is another object of the invention to propose a catalyst carrier having a large surface area of an alumina layer as a catalyst carrying layer and an excellent heat resistance and a method of producing the same.